Four Basic Reaction Types · Intro Chemistry

Why classify reactions?

There are far too many chemical reactions to memorize one by one. But if you just ask “what joins together, what breaks apart, and what swaps places,” most reactions you’ll meet early on fall into four basic reaction types基本反应类型. Once you recognize the type, you can often predict the products.

Think of atoms and groups of atoms as dancers at a party: some pair up, some couples split, a single dancer cuts in and steals someone’s partner, and sometimes two couples switch partners.

The four types at a glance

The colored tiles stand for different elements or groups of atoms. A, B, C, and D can be any of them.

Synthesisalso called combination

A+B→AB

Many → one

Decomposition

AB→A+B

One → many

Single replacementalso called single displacement

A+BC→AC+B

Element + compound → new element + new compound

Double replacementalso called double displacement

AB+CD→AD+CB

Two compounds swap partners

1. Synthesis

Two or more substances form one product. Tell-tale sign: there is only one product.

\ce{C + O2 -> CO2}

\ce{CaO + H2O -> Ca(OH)2}

The last lesson’s $\ce{2H2 + O2 -> 2H2O}$ is also a synthesis reaction.

2. Decomposition

One substance breaks down into two or more products. Tell-tale sign: there is only one reactant.

\ce{2H2O(l) ->[\text{electrolysis}] 2H2(g) + O2(g)}

\ce{CaCO3(s) ->[\Delta] CaO(s) + CO2(g)}

3. Single replacement

An element reacts with a compound to form a new element and a new compound. Tell-tale sign: each side has exactly one element and one compound.

\ce{Fe + CuSO4 -> FeSO4 + Cu}

\ce{Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g)}

Iron can push copper out of copper(II) sulfate solution because iron is more reactive than copper. Which metal can replace which depends on the activity series (coming up in Module 8).

4. Double replacement

Two compounds swap partners to form two new compounds. Tell-tale sign: there are two compounds on each side, and their parts have been exchanged.

\ce{NaOH + HCl -> NaCl + H2O}

\ce{AgNO3(aq) + NaCl(aq) -> AgCl(s) + NaNO3(aq)}

A double replacement reaction usually only happens if it forms a precipitate, a gas, or water (more on this in Module 8).

How to classify: four questions

Count the substances. How many reactants and products are there? Many → one is synthesis; one → many is decomposition.
Look for elements. Is each side “one element + one compound”? Then it’s single replacement.
Look for a swap. Do two compounds exchange parts? Then it’s double replacement.
None of the above? Then it isn’t one of the four basic types.

Note: catalysts and conditions written on the arrow are not reactants.

Example 1

\ce{2KClO3(s) ->[MnO2][\Delta] 2KCl(s) + 3O2(g)}

Step-by-step solution

Find the reactants. The $\ce{MnO2}$ above the arrow is a catalyst and isn’t used up, so the only reactant is $\ce{KClO3}$ .
Find the products. $\ce{KCl}$ and $\ce{O2}$ , two in total.
Conclusion. One → many, so this is a decomposition reaction.

Example 2

\ce{3CO + Fe2O3 ->[\Delta] 2Fe + 3CO2}

Step-by-step solution

Count the substances. Two reactants and two products, so it’s neither synthesis nor decomposition.
Check for single replacement. There’s an element ( $\ce{Fe}$ ) among the products, but both reactants, $\ce{CO}$ and $\ce{Fe2O3}$ , are compounds. Single replacement needs an element among the reactants too, so this isn’t single replacement.
Check for double replacement. One product is an element, so this isn’t two compounds swapping parts either.
Conclusion. It doesn’t fit any of the four basic types. (It’s a redox reaction, which you’ll study in Stage 2.)

Common mistakes

Mistake 1: Calling it single replacement just because an element is produced. Single replacement also needs an element among the reactants. See Example 2.

Mistake 2: Assuming any reaction between two compounds is double replacement. In $\ce{CO2(g) + Ca(OH)2(aq) -> CaCO3(s) + H2O(l)}$ , two compounds react, but they don’t swap parts, so it isn’t a double replacement reaction.

Mistake 3: Thinking every reaction fits one of the four types. $\ce{CH4 + 2O2 -> CO2 + 2H2O}$ fits none of them. Many English-language textbooks give reactions like this their own category: combustion燃烧反应.

Quick check

Practice: classify each reaction

$\ce{2Mg + O2 -> 2MgO}$
$\ce{2HgO(s) ->[\Delta] 2Hg(l) + O2(g)}$
$\ce{Zn(s) + 2HCl(aq) -> ZnCl2(aq) + H2(g)}$
$\ce{BaCl2(aq) + Na2SO4(aq) -> BaSO4(s) + 2NaCl(aq)}$
$\ce{CH4 + 2O2 -> CO2 + 2H2O}$
$\ce{CaO + H2O -> Ca(OH)2}$
$\ce{Cu + 2AgNO3 -> Cu(NO3)2 + 2Ag}$
$\ce{2H2O2(aq) ->[MnO2] 2H2O(l) + O2(g)}$
$\ce{CaCO3(s) + 2HCl(aq) -> CaCl2(aq) + H2O(l) + CO2(g)}$

Summary

Synthesis: many → one, A + B → AB
Decomposition: one → many, AB → A + B
Single replacement: element + compound → new element + new compound, A + BC → AC + B
Double replacement: two compounds swap partners, AB + CD → AD + CB
Not every reaction is one of these four, and catalysts on the arrow don’t count as reactants.

On to AP (for later)

AP Unit 4 (topic 4.7) sorts reactions a different way: acid-base, redox, and precipitation. Every single replacement reaction you saw today is a redox reaction, and double replacement reactions are most often precipitation or acid-base neutralization reactions.

Four Basic Reaction Types四种基本反应类型（化合、分解、置换、复分解）

Learning goals

Before you start

Why classify reactions?

The four types at a glance

1. Synthesis

2. Decomposition

3. Single replacement

4. Double replacement

How to classify: four questions

Example 1

Example 2

Common mistakes

Quick check

Summary

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